Generally, it is old to store the energy from the downstroke of an underground well pump and to utilize the stored energy to assist the well pump in its upward stroke. The present invention is directed to an improved hydraulic pumping unit for actuating an underground pump which utilizes a second piston and an accumulator for storing energy on the downstroke thereby providing a system which reduces the power required, requires less hydraulic operating pressure, allows the pumping unit to be mounted close to the wellhead, and which automatically makes up and limits the air and fluid requirements of the unit.